Structure-activity relationship between prostacyclin and its platelet receptor. Correlation of structure change and the platelet activity.

Correlation analysis between the structural changes of PGI2 and the corresponding changes in platelet activity was performed to look into the following structural features: (1) the spatial relationship of critical functional groups, (2) conformational flexibility of the molecule as a result of chemical modification; (3) charge distribution around C6a; (4) hydrogen-bonding capability and mispairing and (5) the steric effects which resulted from chemical modifications. Our studies led to three important conclusions: (1) The C1 carboxylate and C11, C15 hydroxyl groups of PGI2 are essential for platelet activity. Modifications that change their relative positions reduce the activity. The ring structure and the C5 and C13 double bonds are molecular designs to maintain this unique geometry. (2) The C6a oxygen, although not vital to the binding geometry, is important for the biological potency. Decreasing the electronegativity of C6a oxygen leads to decreased potency. (3) We propose that any additional hydrogen-bond donor present between C1 and C15 could cause a hydrogen-bond mispairing and therefore a decreased activity. These findings should be useful for designing new PGI2 analogues and for determining the configuration of receptor-associated PGI2 by a molecular mechanics technique.